Transmission sensor?

That is called "limp mode" and it allows you to drive slowly to the repair shop without needing a tow truck. Definitely not at 60 mph. It is defaulted to second gear, not third.

There will be a diagnostic fault code stored in the Transmission Computer. Having that read is the place to start. The code will direct you to the circuit or system that needs further diagnosis. You can make some generalizations. If it starts out in second gear as soon as you shift into "drive", it is likely a sensor or some other electrical problem. If it starts out in first gear, then bangs back to second gear during or right after an up-shift, it is usually worn clutch plates or leaking internal seals. The Chrysler scanners and some aftermarket ones will read the "clutch volume index" (CVI). That is a set of four numbers representing the volume of fluid it takes to apply each clutch pack. An experienced transmission mechanic can tell you how much wear is left in each one before a rebuild will be needed.

$25.00 if you replace a sensor yourself to $3000.00 to have the transmission rebuilt at a specialty shop. First you need to find out what's wrong.

Start with the fault codes. Those are stored in the Transmission Computer. Most of the code readers used by the auto parts stores that read codes for you for free only can access the Engine Computer's codes. You need a mechanic with a scanner that will talk to the Transmission Computer.

Also, if the speedometer works properly, the input and output speed sensors should be okay.

Check for a cracked or disconnected vacuum hose to the MAP sensor. Check if the connector got knocked loose for the throttle position sensor.

Which engine do you have? There's three and they're all different. With a computer-controlled transmission you're going to have the 3.0L or the 3.3L.

Is the Check Engine light on while the engine is running?